WiP: Add reboot tool

This PR adds juju-reboot command

Depends on:

https://github.com/juju/juju/pull/707
https://github.com/juju/juju/pull/728
https://github.com/juju/juju/pull/744
https://github.com/juju/juju/pull/782
https://github.com/juju/juju/pull/833
https://github.com/juju/juju/pull/831
again s/Prio/Priority/ throughout